Integrating ecosystem service supply-demand dynamics into watershed management: A novel framework for quantifying...

Understanding the spatiotemporal dynamics of water purification service (WPS) supply-demand mismatches is critical for developing effective watershed management strategies to safeguard downstream water quality in lakes and reservoirs. However, developing a generalizable framework to quantify these complex supply-demand dynamics presents methodological challenges that currently constrain effective water quality management. In the study, we developed a novel framework that quantifies the dynamic WPS supply-demand relationship. By coupling long-term water quality monitoring data with hydrological modeling and random forest algorithms, the framework quantifies the WPS supply capacities of each upstream subbasin and the WPS demand of lakes and reservoirs. We applied this new framework to a critical drinking water source watershed in eastern China. Results showed that (1) Upstream ecosystems retained substantial total nitrogen (TN) and total phosphorus (TP) for the downstream reservoir, intercepting an average of 6.80 × 10⁶ kg TN and 2.57 × 10⁶ kg TP annually, and the supply varied among years, seasons, and subbasins. (2) On the demand side, 14.7 % and 32.4 % reductions in TN and TP loads of the downstream reservoir were required. (3) Temporal mismatches between supply and demand were revealed, with 65 % of TN mismatches concentrated in March-May, and 71 % of TP mismatches occurring in July, March, and June. We recommend spatially targeted conservation measures and temporally adaptive management strategies for optimizing WPS supply-demand balance. Our framework provides an effective method for giving information to targeted watershed management by resolving spatiotemporal disparities of WPS supply and demand.